Her father died aged 56 in his sleep on the estate when the Queen was just 25.

She was on a trip to Kenya at the time, having to cut the tour short and fly back to the UK.

The royals on their way to Sandringham in 2017. (Image: Joe Giddens/PA Wire/PA Images)

The Queen wrote a heartfelt letter to her father's private secretary revealed her deep sorrow at his early death.

A young Queen Elizabeth wrote to Sir Eric Mievill a month after she became Queen, saying: “It all seems so unbelievable still that my father is no longer here and it is only after some time has passed one begins to realise how much he is missed.”

Handwritten on March 3, 1952, the letter was one of the first Elizabeth finished with her new signature as Queen – “Elizabeth R’”.

She wrote she was “thankful” her father died so peacefully and how husband Philip and children Charles, three and Anne, 18 months, were helping with her grief.